We usually recommend starting around 8am/9am. This is totally up to you. In the summer months you might want to start earlier to avoid the heat, also daylight hours are longer.
Walking times are based on real time assessment as per a normal walker (4km/h and/or 300m/h ascent and/or 400m/h descent) and exclude breaks.
You want to be walking between 4 and 6 hours ideally, excluding breaks.